Mistakes That Doomed a Jet Are Crash's Biggest Mystery

By RICHARD WITKIN

Published: January 1, 1996

Correction Appended

The crash of an American Airlines jet near Cali, Colombia, on Dec. 20 has confounded pilots and aviation experts like few other accidents in recent years.

How could the pilots, with years of experience and a cockpit full of electronic navigation gear, have lost track of their location relative to a critical radio beacon just 33 miles north of the Cali airport, their destination?

Was there some reason they did not, or could not, follow the symbol of their Boeing 757 on the moving-map display in front of them? Why were the pilots unaware of their aircraft's position on the basis of data from other electronic devices?

When they instructed their computer to steer the airliner to the beacon and realized it was turning toward known mountains to the east, why did they not make an emergency climb to an altitude above any peaks? Why did they continue instead to lose altitude while in this turn, and in a subsequent turn to take them back on course?

These were some of the prime questions being repeatedly asked by pilots and Government and industry officials in the United States after Colombian authorities made public initial data from the flight recorders recovered from the wreckage.

Seldom in recent years has the aviation world been so thunderstruck by the chain of events combining to produce an air disaster. Seldom has largely inexplicable pilot error seemed to figure so prominently in the cause, whatever other contributions may have been made by gaps in radar, confusion in air-to-ground communications or routine technical aberrations. Initial findings have contained no evidence of sabotage, serious mechanical problems or critical malfunctions of navigation gear on the ground or in the plane.

The crash, in which 160 of the 164 people on board were killed, was the worst for an American carrier since the terrorist bombing of a jumbo jet over Lockerbie, Scotland, in 1988. The plane model involved, the twin-jet 757, had had no accidents in the 13 years since it was introduced. It has become a favorite of pilots, not least for the advanced electronics that make possible impressive feats of navigation and control.

"Like other airliners of this generation, the plane is flown for the most part by the flight management computer," said Cy Cyganiewicz, an airline captain who has flown Boeing 757's. "And the pilot is essentially a systems manager who mainly has to monitor that it's doing what it's supposed to."

This is a widely shared view. But many safety specialists are quick to add the caveat that, with the progress of automation, pilots -- as well as controllers, mechanics and operations people -- can be lulled into complacency, which can readily lead to accidents.

"A form of complacency we seem to be looking at here is a crew's loss of the big picture, sometimes called situation awareness," said C. O. Miller, an internationally respected safety consultant. "Loss of awareness is too often compounded by excessive dependence on these automated systems, especially when there is only a short time to act."

Here is how the airliner would normally have been operated on its trip and what initial analysis of flight-recorder data indicates led to the disaster:

Before takeoff from Miami, the flight management computer was programmed to guide the plane on a standard route, taking it past a succession of radio-navigation stations, or fixes, on the ground. The computer system would automatically tune in successive stations. The plane's automatic pilot would steer the plane either directly over stations, or off to one side at specified distances, or over the intersection of signals from two stations.

The pilots' role was to use radios and dashboard instruments to verify that the automatic system was taking them where it was supposed to. They also would radio controllers, keeping them updated on the plane's location.

The primary ground stations are known as V.O.R.'s, for "very high frequency omnidirectional range station." These send out radio signals so a crew will know the precise direction from their plane to the station. More advanced stations also have distance-measuring equipment that shows on a cockpit instrument the miles from plane to station.

The flight, on a clear but moonless night, seemed problem-free until the southbound plane neared the V.O.R. station named Tulua.

Controllers at Cali had cleared the pilots to fly to the Cali V.O.R., just south of the airport, where radar destroyed by guerrillas had never been replaced. The pilots were to report passing Tulua and to keep to an altitude of 15,000 feet. That was vital because the assigned route went down a valley between treacherous Andes peaks.

The pilots never did report reaching Tulua, and the station appears to be a fateful link in the events that led to the crash. Two minutes after the pilots acknowledged that they would report at Tulua, controllers radioed that the wind at Cali was calm and asked the crew if they could land from the north.

The crew agreed but added: "We'll need a lower altitude right away," since coming straight in would give them less time to get to runway level.

Now the crew was cleared to follow a standard arrival procedure from the north called "Rozo No. 1," and again was reminded to "report Tulua." But the station, which did not have the distance-measuring device, continued to prove elusive.

The pilots consulted landing charts and talked about the navigation aids to be used for a "Rozo No. 1" landing, particularly where such aids were situated in relation to Tulua.

The crew punched in computer instructions to guide them to Tulua. Promptly, the autopilot started a left turn to the east, toward mountainous terrain. Apparently without the crew's realizing it, the plane had already passed south of Tulua and was turning back.

That was not what the pilots wanted. And they quickly called controllers seeking permission to get back to the path to the runway. Specifically, they requested and were given approval to fly directly to the Rozo beacon, just 2.6 miles north of the runway. That meant rolling the plane from its left turn to a right turn. But during both turns, the pilots inexplicably allowed the plane to keep losing altitude.

Nine seconds before the end of the voice-recorder tape, a warning sounded: "Terrain, Terrain. Pull Up." They did. But they could not clear the ridge.

Maps shows the site of crash.

Correction: January 6, 1996, Saturday An article on Monday about an American Airlines jet crash in Colombia mistakenly suggested that computers on the plane, a Boeing 757, relied on successive ground-based radio beacons to navigate. In fact, the computers used the beacons only to refine the course for which they had been programmed before takeoff and for which they would need no contact with navigational aids on the ground.